linux-kernel - Re: [PATCH 6/6] vduse: Update api version to 1

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Date:   Wed, 29 Jun 2022 05:22:47 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        Liu Xiaodong <xiaodong.liu@...el.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] vduse: Update api version to 1

On Wed, Jun 29, 2022 at 05:02:40PM +0800, Yongji Xie wrote:
> On Wed, Jun 29, 2022 at 4:33 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Wed, Jun 29, 2022 at 04:25:41PM +0800, Xie Yongji wrote:
> > > Let's update api version to 1 since we introduced
> > > some new ioctls to support registering userspace
> > > memory for IOTLB.
> > >
> > > Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> >
> >
> > Adding new ioctls does not justify things like this.
> >
> 
> What I want to do here is make userspace know whether this feature is
> supported or not in the kernel. So do you think we need to add
> something like CHECK_EXTENSION ioctl here?

Why bother? unsupported ioctls just return an error code.

> > Besides, adding UAPI then changing it is not nice
> > since it makes git bisect behave incorrectly.
> >
> > > ---
> > >  drivers/vdpa/vdpa_user/vduse_dev.c | 12 ++++++++++++
> > >  include/uapi/linux/vduse.h         |  8 +++++++-
> > >  2 files changed, 19 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > index 7b2ea7612da9..2795785ca6a2 100644
> > > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> > > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > @@ -1206,6 +1206,10 @@ static long vduse_dev_ioctl(struct file *file, unsigned int cmd,
> > >       case VDUSE_IOTLB_GET_INFO: {
> > >               struct vduse_iotlb_info iotlb;
> > >
> > > +             ret = -EPERM;
> >
> >
> > Almost for sure a wrong error code.
> >
> > > +             if (dev->api_version < 1)
> > > +                     break;
> > > +
> > >               iotlb.bounce_iova = 0;
> > >               iotlb.bounce_size = dev->domain->bounce_size;
> > >
> >
> >
> > Wait a second. so you are intentionally breaking any userspace
> > that called VDUSE_SET_API_VERSION with version 0?
> >
> > Please don't.
> >
> 
> Yes, I'd like to let userspace know we don't support this feature.
> 
> Thanks.
> Yongji


-- 
MST